1. Field of the Invention
The subject invention generally pertains to HVAC controls that receive feedback signals from several thermodynamic sensors and more specifically to protecting the control""s current loop inputs that receive sensors"" feedback.
2. Description of Related Art
HVAC equipment (heating, ventilation and air conditioning equipment), such as heaters, air conditioners, heat pumps, blowers, humidifiers, dehumidifiers and VAV valves (variable air volume valves) typically condition the environment of a room or area within a building in response to sensors or transducers that sense a thermodynamic condition (e.g., temperature, pressure, humidity, etc.) of the area""s environment. With generally large areas, as is the case with many commercial buildings, the area is often divided into separately conditioned comfort zones with each zone having its own sensor (The terms, xe2x80x9csensorxe2x80x9d and xe2x80x9ctransducerxe2x80x9d are being considered as equivalent and interchangeable).
Since control wiring is often low voltage, the sensors are often serviced while the control and the remainder of the HVAC system is still operating. The supply voltage is usually around 24 volts, and feedback signals from conventional sensors are usually no more than 5 volts or 4 to 20 mA. Leaving the control power on while servicing a sensor not only benefits the occupants of the area being conditioned but is also more convenient for an electrician doing the servicing. Unfortunately, such a practice can create a current overload problem if the electrician inadvertently shorts the sensor and briefly misapplies a full 24 volts across the control""s input terminals.
Of course, conventional current limiting practices can be used to address this problem. For example, the control circuit could simply include a conventional fuse or circuit breaker. However, it is difficult to properly size a fuse for a generic, universal control for an indeterminate number of sensors that will be field-connected to the control. More specifically, a common fuse would need to handle the sum of the current delivered to all the sensors. Consequently, each control input would need to have more current carrying capacity than the fuse in order for the fuse to protect an individual input. Moreover, it would be a nuisance to have a brief inadvertent short cause an entire HVAC system to shutdown until manually reset.
Another approach is to simply size the control""s input components to handle the excess current from a shorted sensor. But such an approach obviously adds cost and bulk to the control. The problem becomes worse when compact surface-mount input components are used in an attempt to reduce cost and bulk, as one loses the heat sink benefit previously provided by the electrical leads of components having such leads.
To protect the input components of an HAVC control system, it is an object of the invention is to limit the maximum voltage and current that can be applied to such input components and further disable the electrical power source should the voltage and current limits be reached.
Another object of the invention is to provide an input overload protection system that automatically and periodically attempts to reset itself after responding to an overload fault.
Another object is to provide an input overload protection system that can be used with an indeterminate number of sensors.
A further object is to provide an input overload protection system that is compatible with a variety of thermodynamic transducers including those that sense temperature, pressure and humidity.
A still further object of the invention is to provide an input overload protection system that allows the use of input components that are smaller and less expensive.
Yet another object is to provide an input overload protection system that allows the use of input components that have lower current carrying capacity.
Another object is to provide an input overload protection system that allows the use of input components that can be surface mounted rather than having to rely upon the heatsinking property of through-hole soldered leads.
These and other objects of the invention are provided by an input overload protection system that includes several inputs. The voltage that a current feedback signal can apply to the inputs is limited to a predetermined maximum voltage by a voltage limiter at each input. An electrical supply monitor disables a power source upon detecting that the predetermined maximum voltage has been reached.